This principally relates to a so-called distributor-type fuel-injection pump. In the case of such solenoid-valve-controlled injection pumps that are preferably used in diesel engines, the injection period is controlled by the opening of the solenoid valve. In order that the diesel engine achieves good emission figures, the pressure in the line decreases as rapidly as possible. This can only be achieved by a quick-opening solenoid valve. Solenoid-valve-controlled pre-injection is only possible, using a quick-opening solenoid valve. Therefore, the solenoid valve is constructed in such manner, that its opening time can be reduced by hydraulic forces.
Particularly relevant to the present invention are those distributor-type fuel-injection pumps, in which a so-called I-solenoid valve is used. This type of valve construction distinguishes itself in that, in response to deactivation, the flow is radially directed from the outside to the inside. An opening (positive) force is achieved by diverting the flow in the low-pressure range. The force shortens the valve opening times.
A basic characteristic of the type of valve in question is a low-pressure surface, which is very large in comparison with the pressure-stage surface in the high-pressure region of the solenoid valve. Therefore, relatively large forces already occur in response to small pressure fluctuations in the low-pressure region. These forces cause fluctuations in the opening time, which result in deviations in the injection amount (from stroke to stroke). In order to at least partially compensate for the mentioned forces and thus largely prevent their disadvantageous effects, the valve type in question is provided with a low-pressure compensating piston, which interacts with the solenoid valve. Therefore, the low-pressure compensating piston has the function of producing stable opening characteristics of the solenoid valve. German Published Patent Application No. 4339948, whose subject matter is a fuel-injection pump of the species, belongs to the above-described related art. In the known fuel-injection pump, the low-pressure compensating piston is constructed in one piece with the solenoid-valve needle and positioned coaxially to it, it being practically a continuation of the solenoid-valve needle beyond the valve seat.
The disadvantage is that, upon terminating fuel delivery, flow is diverted on the low-pressure compensating piston. This diversion of the flow causes a pressure increase that creates a closing needle force. The disadvantageous effect is a delay in the opening of the solenoid valve during fuel-delivery termination.
The object of the present invention is to take appropriate measures to prevent unwanted delays in opening the solenoid valve.